Preserve mixed evaluation evidence with reserved atomic report checkpoints

tools/mixed-eval/test_evaluate.py

@@ -3,12 +3,38 @@ import importlib.util
 from pathlib import Path
 import tempfile
 import unittest
+from unittest.mock import patch
 
 spec = importlib.util.spec_from_file_location('mixed_eval', Path(__file__).with_name('evaluate.py'))
 evaluate = importlib.util.module_from_spec(spec); spec.loader.exec_module(evaluate)
 
 
 class EvaluationTests(unittest.TestCase):
+    def test_existing_report_is_preserved_and_new_report_is_reserved(self):
+        with tempfile.TemporaryDirectory() as directory:
+            path = Path(directory) / 'nested' / 'report.json'
+            evaluate.reserve_report(path)
+            evaluate.write_report(path, {'complete': False, 'query': 'Straße'})
+            previous = path.read_bytes()
+            with self.assertRaises(FileExistsError):
+                evaluate.reserve_report(path)
+            self.assertEqual(path.read_bytes(), previous)
+            evaluate.write_report(path, {'complete': True})
+            self.assertIn('true', path.read_text(encoding='utf-8'))
+            self.assertEqual(list(path.parent.glob('*.partial')), [])
+
+    def test_failed_checkpoint_publication_keeps_previous_report(self):
+        with tempfile.TemporaryDirectory() as directory:
+            path = Path(directory) / 'report.json'
+            evaluate.reserve_report(path)
+            evaluate.write_report(path, {'complete': False, 'rows': [1]})
+            previous = path.read_bytes()
+            with patch.object(evaluate.os, 'replace', side_effect=OSError('disk failure')):
+                with self.assertRaises(OSError):
+                    evaluate.write_report(path, {'complete': True})
+            self.assertEqual(path.read_bytes(), previous)
+            self.assertEqual(list(path.parent.glob('*.partial')), [])
+
     def test_frozen_real_assets_verify(self):
         fixture, _ = evaluate.load_fixture()
         self.assertEqual(len(fixture['assets']), 32)